Vacuum processing systems are well known in the art and used for a variety of applications, e.g. for the production of thin-film solar cells or TFT displays in the photovoltaic industry or in the display industry, respectively. In general a vacuum processing system comprises a transport path for substrates in a vacuum environment. Along said transport path various processing means (processing stations or modules) may be arranged to act on the substrate, e.g. heating means, cooling means, means for layer deposition by, inter alia, chemical vapor deposition (CVD), means for etching or quality control, and so on. EP 0 575 055 and U.S. Pat. No. 4,358,472 show, so called, inline vacuum processing systems. In general, the modules of the vacuum processing system are separated from each other by valves or gates in order to avoid cross-contamination and the pressure within said modules is set below ambient, i.e. atmospheric pressure by means of vacuum pumps such as fore vacuum pumps or high vacuum pumps.
During the operation of the vacuum processing system, processing gases are fed into the processing environment, e.g. diethyl zinc (DEZ) into a PECVD deposition module. Furthermore, during deposition the process gases are depleted and a permanent flow of fresh process gas is fed into the processing system. At the same time, the vacuum pumps are operated permanently, to keep up the desired process pressure. Consequently, process gas comprising reaction by-products and/or unreacted reagents is continuously evacuated/exhausted from the vacuum processing system.
However, the reaction by-products and/or unreacted reagents tend to aggregate together under conditions prevalent in the module exhaust, the vacuum pump and the equipment piping, which may lead to negative effects on the process efficiency, blockage of the piping and vacuum pumps, reduction of production cycle time and/or the need for frequent cleaning cycles. Especially, the vacuum pumps are affected since the pump increases the local pressure from process pressure up to atmospheric pressure and heats up the gas due to the compression. Both effects result in lead to an increased reaction of the process gases inside the vacuum pump which may lead to metallic or oxidic deposition (e.g. Zn or ZnO from diethyl zinc) which, in turn, result in reduced lifetime of the pumps, a clogging of the vent lines etc. This problem is even more pronounced during production in industrial scale, where high throughput of substrates requires a high consumption of process gases.
In conventional vacuum processing systems a trap is placed upstream of the vacuum pump to clean the exhausted gas from reaction by-products and the unreacted reagents. Hot-traps and cold-traps are known in the art. Cold-traps allow condensation or recombination of gas constituents to remove or inactivate parts of the unused process gases and/or reaction by-products. However, cold-traps will be saturated very fast and may lead to a concentration of a single component in the exhaust stream which might be a health hazard, requiring additional safe handling requirements.
Various designs of hot-traps are known in the art which either use mechanical constrictions in the path of gas flow or an enlarged surface area. These premises are based on the fact that the reaction of components present in the process gas is surface-based. Thus, this surface can either be provided by mechanical design, or created when reaction products from previous cycles are deposited within the equipment, e.g. the piping.
Conventional hot-traps require extensive and frequent cleaning and/or replacement which, due to the construction of the hot-trap, requires a considerable amount of time. This, in turn leads to a reduced uptime of the vacuum processing system and high maintenance costs. More importantly, with conventional hot-traps the reaction by-products and unreacted reagents flow through the hot-trap in a laminar fashion which results in an inhomogeneous concentration of said substances within the gas stream as no or little mixing occurs. Consequently, conventional hot-traps only provide sub-optimal reaction conditions. In addition, the concentration of reaction by-products and the unreacted reagents decreases within the hot-trap in the direction of the gas flow resulting in a change of optimal reaction parameters. Thus, conventional hot-traps do not offer ideal reaction conditions over their full length.